Typically, in bubble jet print heads a plurality of electrically resistive heater elements are formed on a support substrate, e.g. formed of metal or ceramic material and having a heat control coating e.g. SiO.sub.2. Metal electrodes are formed to selectively apply voltage across the heater elements and a protective coating is provided over the heater elements and electrodes. Printing ink is supplied between the heater elements and orifices of the print head and a heater is energized to a temperature that converts the adjacent ink to steam rapidly, so that a shock wave causes ejection of ink from the related orifice.
This ink jet printing approach is becoming increasingly useful; however, a major problem still exists in providing print heads wherein the heater elements are capable of a long operative life, particularly when used in high speed printing modes. Primarily, this is because protecting the drop ejectors from physical and chemical damage still presents a major technical problem.
Thus, the inks that are utilized can chemically attack the heater elements and effect short-circuits between their address and ground electrodes. More specifically, the resistor is an electrically energized device and the ink is an electrolyte. Any device that causes an electric current to flow through an electrolyte will cause electrolytic dissolution at the positive electrode and electrolytic plating at the negative electrode. Therefore the resistor will tend to be dissolved at the positive end, while having electrolytic material deposited at the negative end, unless the resistor is shielded from the electrolyte. For these reasons, and other reasons, e.g. protection against mechanical damage, a dielectric protective layer(s) are provided over the heater element (and usually over the electrodes). The provision of such dielectric protective layers over the heaters reduces problems such as mentioned above, but introduces additional difficulties, e.g. in regard to the efficient transfer of heat from the heater to the ink and the reliable attachment of such layers to the heater and electrodes.
U.S. Pat. Nos. 4,450,457 and 4,577,202 describe the above and other problems and provide some exemplary listings of desired protective layers characteristics, for example having a good resistance to heat and ink damage, having a good heat conductivity, having an ink penetration preventive property, having an oxidation preventing property and having a resistance to mechanical damage. To achieve such characteristics, the noted patents teach use of a two layer composite protective cover comprising a dielectric. e.g. SiO.sub.2 or immediately over the heater element and a metal layer e.g. Ta or metal alloy, as the top layer. U.S. Pat. No. 4,513,298 describes another composite protective layer construction using silicon nitride as the first overlying layer, but using silicon carbide as the top protective layer.
While the protective layer approaches described above are useful, they still are subject to operative failure in less than the desired life span.